The Institute of Electrical and Electronic Engineers (IEEE) has recently been proceeding with standardization of a wireless communication system known as Worldwide Interoperability for Microwave Access (WIMAX).
WIMAX consists of IEEE standard 802.16d targeted at stationary subscriber stations and IEEE standard 802.16e targeted at mobile subscriber stations (mobile stations: MS).
FIG. 1 is a drawing depicting an example of the network configuration of a wireless communication system. The network configuration depicted in FIG. 1 is composed of a core network 1, access network AN, which includes a control device 2 and a plurality of wireless base stations BS3-1 to BS3-3 connected to the control device 2 (to be simply referred to as wireless base stations BS unless specified individually), and a mobile station MS.
Furthermore, although the control device 2 is depicted independently from the plurality of wireless base stations BS in the system, the system can also be configured so that the function of the control device 2 is provided within any one of the plurality of wireless base stations BS. Thus, the control device 2 can be understood within the concept of being a wireless base station side device.
A mobile station MS in the idle mode is not registered in units of specific wireless base stations BS, but rather monitors incoming notification messages (paging messages) 100 transmitted by all wireless base stations within paging groups PG (also referred to as a paging area, location area or location registration area) 4-1, 4-2 and 4-3 (to be simply referred to as paging groups PG unless specified individually), and confirms the presence of traffic addressed to the mobile station MS at a fixed cycle.
Since frames targeted for monitoring of incoming notifications are defined intermittently, a mobile station MS is able to reduce power consumption by pausing reception during frame periods of frames other than the frames targeted for monitoring of incoming notifications.
Since a mobile station MS in the idle mode does not register terminal information to a specific wireless base station BS, as long as the mobile station MS is moving within the same paging group PG, it is not necessary to carry out re-registration even if the mobile station MS has moved between wireless base stations BS.
In the case a mobile station MS in the idle mode moves to a different paging group PG (for example, from paging group PG4-1 to paging group PG4-2), the mobile station MS in the idle mode carries out location registration updating processing and registers the paging group PG to which it currently belongs in the control device 2.
The control device 2 has a paging controller and a location register, manages a plurality of wireless base stations BS, and manages to which paging group PG a mobile station MS in the idle mode belongs. When the control device 2 receives incoming data for a mobile station MS in the idle mode, it transmits an incoming notification (paging announce) to a plurality of wireless base stations BS within the paging group PG to which the mobile station MS belongs.
After having received this incoming notification, the wireless base stations BS advertises (broadcast or multicast) an incoming notification message 100 (paging message: MOB_PAG-ADV).
When the mobile station MS confirms that the incoming notification message 100 is addressed to itself, it leaves the idle mode IM and switches to a normal mode NM. It then registers with a specific wireless base station BS and establishes a link with that specific wireless base station BS.
After establishing a link, the wireless base station BS performs call reception (or calling) to the relevant mobile station MS by transmitting a signal addressed to that mobile station to the corresponding mobile station MS.
FIG. 2 is a drawing depicting an idle mode transition procedure of the prior art. A mobile station MS desired transition to the idle mode IM transmits an idle mode transition request message in the form of a (DREG-REQ: De-REgistration REQuest) to a wireless base station BS (Step S1). At this time, the mobile station MS includes a paging cycle request in the idle mode transition request message DREG-REQ.
The paging cycle request is a request for a paging cycle (PC) desired by the mobile station MS, or in other words, an intermittent reception cycle desired by the mobile station MS.
Although a short paging cycle PC makes it possible to shorten the response delay of the mobile station MS to an incoming signal, this also results in frame targeted for monitoring of incoming notifications occurring more frequently, thereby increasing the power consumption of the mobile station MS.
Conversely, although a long paging cycle PC results in a longer response delay, since frames targeted for monitoring of incoming notifications do not occur that frequently, power consumption of the mobile station MS decreases.
The mobile station MS determines its own paging cycle request in consideration of a tradeoff between response delay and power consumption.
A wireless base station BS that has received an idle mode transition request message (DREG-REQ) transmits that it has received the idle mode transition request to the control device 2 using an idle mode transition request reception confirmation message in the form of an (IM_Entry_State_Change_Req) (Step S2).
At this time, the wireless base station BS includes a mobile station identifier (MSID), a wireless base station identifier (BSID) and a paging cycle request in the idle mode transition request reception confirmation message (IM_Entry_State_Change_Req).
When the idle mode transition request reception confirmation message (IM_Entry_State_Change_Req) is received by the control device 2, the control device 2 determines the paging cycle PC, paging offset Poff, paging interval length (PL) and paging group PG of the mobile station MS after considering the paging cycle request, and then retains the paging cycle PC, paging offset Poff, paging interval length PL and paging group PG of the mobile station MS.
The paging offset Poff indicates the location in the paging cycle PC where the frame targeted for monitoring of incoming notification starts.
Although to subsequently explained again in detail, the paging interval length PL is a period within the paging cycle PC from the start to the end of the frame targeted for monitoring of incoming notification.
With the exception of cases in which the paging cycle of the paging cycle request is extremely large or extremely small, for example, the control device 2 basically uses the same value as the paging cycle request for the determined paging cycle PC.
The control device 2 then transmits an idle mode transition request reception response message (IM_Entry_State_Change_Rsp) to the wireless base station BS that further includes the mobile station identifier (MSID), base station identifier (BSID), paging cycle PC, paging offset Poff, paging interval length PL and paging group PG (Step S3).
When the idle mode transition request reception response message (IM_Entry_State_Change_Rsp) is received, the wireless base station BS transmits an idle mode transition command message in the form of (DREG-CMD: De-REgistration CoMmanD) to the mobile station MS (Step S4).
At this time, the wireless base station BS includes the paging cycle PC, paging offset Poff, paging interval length PL and paging group PG in the idle mode transition command message (DREG-CMD).
Subsequently, the mobile station MS switches from the normal mode NM to the idle mode IM in which it intermittently monitors frames targeted for incoming notification monitoring defined by the received paging cycle PC, paging offset Poff and paging interval length PL.
Furthermore, although the paging interval length PL of the mobile station MS is determined by the control device 2 in FIG. 2, it may also be determined by a wireless base station BS. In the case of using a paging interval length uniquely given in the system instead of determining paging interval length PL for each mobile station, instead of the control device 2 transmitting a paging interval length PL determined for each mobile station to the wireless base stations BS, the wireless base stations BS may transmit the given paging interval length PL to a mobile station MS.
Although the specifications for determining paging interval length PL of mobile stations MS as described above are not particularly limited to any of the forms in IEEE standard 802.16e, current specifications of the WIMAX Forum NWG (Network Working Group) specify determination by the wireless base stations BS.
A mobile station MS carries out an idle operation at a paging cycle PC determined on the basis of a desired paging cycle request according to the idle mode transition procedure as described above.
On the other hand, the control device 2 retains the paging group PG, paging cycle PC, paging offset Poff, and depending on the implementation method, paging interval length PL of the mobile station MS.
Paging cycle PC, paging offset Poff and paging interval length PL are defined in frame units according to IEEE standard 802.16e, and frames targeted for incoming notification monitoring are defined as depicted in FIG. 3 in accordance with the following equation using these parameters.Nframemodulo paging cycle PC=paging offset Poff
Here, “modulo” is a remainder operator.
Nframe is the frame number of the starting frame of those frames targeted for incoming notification monitoring (112 and 122 in the example depicted in FIG. 3). Those frames present over the paging interval length PL starting from this starting frame are targeted for incoming notification monitoring.
An incoming notification monitoring period I is generated for each number of frames of a paging cycle (10 frames in FIG. 3). When the wireless base stations BS broadcast an incoming notification message 100, it broadcasts the message 100 with any of the frames targeted for incoming notification monitoring. Frame length is defined on the basis of time in milliseconds, for example.
Moreover, paging offset Poff is controlled so as to be able to be changed. This is to disallow incoming notification messages to a mobile station MS to concentrate in a specific frame. In the example of FIG. 3, in the case of setting the paging cycle PC to 10 frames and generating an incoming notification monitoring period I for each number of frames of the paging cycle PC, a paging offset Poff equal to two frames is imparted, the incoming notification monitoring period I starts from the 112th frame, and the paging interval length PL is 2 frames.
A rest period RC is present after the incoming notification monitoring period I.
In IEEE standard 802.16e, a mobile station MS in the idle mode IM may receive a paging cycle PC, paging offset Poff and paging interval length PL simultaneous to being notified of a new paging group PG by the control device 2 by location registration updating processing.
This is to notify the mobile station MS of changed values in the case the control device 2 desires to change the paging cycle PC, paging offset Poff and paging interval length PL according to a paging group PG.
The control device 2 subsequently retains the changed paging cycle PC, paging offset Poff and paging interval length PL.
In addition, there are also technologies of the prior art for changing the paging group PG (also referred to as paging area, location area or location registration area), or changing the paging cycle PC according to differences in time periods (such as day or night) or differences in services subscribed to by the mobile station MS (see, for example, Patent Documents 1 to 3).    Patent Document 1: Japanese Laid-open Patent Publication No. 2004-247801    Patent Document 2: Japanese Laid-open Patent Publication Laid-open No. 2005-79660    Patent Document 3: Japanese Laid-open Patent Publication No. H6-141365